Since then, many studies looked for FG's progenitors, using a variety of techniques. [1]} studied a sample of 25 clusters in the X-ray, finding that some of them are dimmed in luminosity. Their interpretation of the result was that, according to a specific toy simulation, groups with dimmed X-ray luminosity have lower velocity dispersion. This would lead to the formation of FGs, since low-velocity encounters between massive galaxies are the most efficient in terms of merging time scale. This result was confirmed also by the analysis of a compact group at \(z=0.22\)  studied in [2]}. In fact, these authors showed that this system has many characteristic in common with FGs and, in particular, the merging of the four brightest members would lead to the formation of a BCG of \(M_r \sim -23\) , a typical value of central galaxies in FGs. A similar result was also found in the study of galaxy pairs [3]}. In this case, the author claims that E+S pairs could be the last step in the formation of FGs, thus a sort of transition between compact groups and FGs. Also [4]} found evidences that compact groups are favoured as progenitors over the early assembly. On the other hand, [5]} found a M/L ratio for FGs that is too high if compared with compact groups, thus claiming that these systems are not the ideal progenitors.
